The invention generally relates to welding, brazing, metal fusion and the like (hereinafter "welding"), and in particular to a control system for welding that operates by means of infrared thermography.
In the welding of two metal plates disposed end-to-end with the use of a welding head which generates a high energy-density welding beam, e.g. a laser beam or electron beam, there are essentially three phases of the operation:
temperature rise in the material; PA1 fusion of a certain amount of the metal; and PA1 cooling of the welded joint and environs. PA1 a longitudinal thermal plot; PA1 a transverse profile; and PA1 a three-dimensional thermal surface. PA1 La Thermographie Infrarouge, pub. Technique et Documentation Lavoisier, 3rd Ed. 1989; PA1 Capteurs Infrarouges: Le Soudures Analysees En Temps Reel--Infrared Detectors--Real-Time Analysis Of Welding, in the journal Mesures, Jan. 19, 1987. PA1 longitudinal position along the axis of the weld (along the plane of the joint); PA1 position transverse to said axis; and PA1 focusing of the optics of the camera. PA1 a welding head which generates a high energy-density welding beam, e.g. a laser beam or electron beam, and PA1 a camera for thermographically monitoring the welding being carried out by means of said beam, which welding head and camera are both supported by a frame member; wherewith, in the subject method, PA1 in a first step, the welding beam is moved into a reference position with respect to a point source of light disposed in a zone of the welding apparatus or installation, which zone is accessible by the welding beam, which reference position is in a plane P close to or coincident with the "median" plane between the two planes containing the respective principal faces of the metal plates which faces are directed generally toward the welding head and the camera; PA1 in a second step, the frame member is displaced in the direction of the plane of the joint by a specific distance d which corresponds to the distance desired in said direction between: PA1 the impact point of the welding beam on the metal plates being welded and PA1 the field of view of the camera on said plates; PA1 in a third step, the field of view of the camera is adjusted such that the camera registers signals from the aforesaid light source; PA1 in a fourth and final step, the camera is locked in position with respect to the frame member. PA1 In the first step, the welding beam is moved into a reference position by means of a low power laser light beam visible to the eye, which low power beam is coaxial with the welding beam and serves as an aiming or aligning beam. PA1 If the high energy-density welding beam is a laser beam, during the first step one positions said welding beam in a reference position by means of said welding beam itself, namely by coupling said beam with a device which renders said beam visible. PA1 A light source apparatus is chosen which has a diaphragm and which gives rise to light with "isotropic" properties at least in the directions directed toward the camera. PA1 In the first step, the manner in which the welding beam is positioned in a reference position with respect to the light source is to direct the visible-light aiming or aligning laser beam, or the welding beam, toward the diaphragm of the light source apparatus. PA1 In the first step, the manner in which the welding beam is positioned in a reference position with respect to the light source is to direct the visible-light aiming or aligning laser beam, or the welding beam, toward a point O the spatial coordinates of which are controlled with respect to the spatial coordinates of the diaphragm of the light source apparatus. PA1 The field of view of the camera is adjusted by displacing said camera and/or by electronic adjustment of said field of view ("windowing"). PA1 In an intermediate step prior to locking the camera in place, the focal length of the camera regarding the light source is adjusted. PA1 a welding head which generates a high energy-density welding beam, and PA1 a camera for thermographically monitoring the welding being carried out by means of said welding beam, which welding head and camera are both supported by a frame member; PA1 characterized in that said apparatus is comprised of: PA1 means of displacement of the frame member; PA1 a fixed light source disposed in a zone of the welding apparatus or installation, which zone is accessible by the welding beam; and PA1 means for geometrically regulating the position of the camera with respect to the welding head. PA1 the light source is comprised of: PA1 a cavity of a cylindrical or spherical shape, and PA1 a diaphragm formed by an opaque plate having an aperture with diameter less than 1 mm, wherewith the thickness of the opaque plate at said aperture is less than 0.3 mm; and PA1 the means of geometrically regulating the position of the camera with respect to the welding head include at least one micrometer stage or the like.
The time, spatial location, and temperature are the three variables which define the thermal aspects of the process; i.e., for fixed conditions of welding, at a given instant a point disposed in the zone being welded has a given "thermal level" giving rise to IR emissions, and representing enthalpy or the like.
Accordingly, analysis of the thermal image of the weld during the welding process enables one to monitor the quality of the welded joint and to control the quality of the welding process, in real time, amid changing variables.
It is known to achieve such monitoring and control using an infrared-sensitive camera (IR camera) which indicates the thermal profile prior to, during, and/or after the fusion.
Depending on the type of information sought, the IR camera may observe the zone immediately ahead of the advancing zone of fusion; this enables one to have, e.g., a thermal image which can be used for guiding the welding head. The trough in the thermal profile transverse to the plane of the joint represents the gap separating the two metal plates to be welded together. The IR camera may also observe the thermal image at the fused mass of metal in order to indicate the temperature at the surface of the metal, and the width of the molten mass. Or the IR camera may observe the zone behind the fusion zone so as to determine the depth of penetration and variations in the welding process.
Three forms of visual display may be provided for the thermal images of the zone observed by the IR camera along the length of the weld:
The IR cameras used in this type of application may be of a scanning type wherein the elementary field is moved by optomechanical means or electronic means. Optomechanical means include complete rotation of a system, oscillation of a plane mirror, spinning rotation of a drum bearing a polygonal mirror (i.e. a mirror in the shape of a polygonal parallelepiped), rotation (e.g. spinning rotation) of a refractive polygonal prism, rotation of a drum bearing lenses, use of a diaporameter, and use of a linear bank of detectors. The latter arrangement using a linear bank of detectors has the advantage of not requiring mechanical movements. One may also employ a "matrix camera".
The aforesaid known method of monitoring welds and controlling the welding process with the use of IR thermography is described in the following publication articles:
In order to be able to make practical use of the images produced by the IR camera for monitoring welds and controlling the welding process in view of the high temperature gradients present in welding (which are typically several hundred degrees Centigrade per millimeter), one must have full and accurate control over the position of the field of view of the camera with respect to the point of impact of the welding beam, in terms of:
If one does not perfectly control the positioning of the field of view of the camera with respect to the point of impact of the welding beam, it will be difficult to interpret the thermal plots obtained because they will not represent the field supposed but rather a neighboring zone.
Thus, e.g., if the field of view of the camera is theoretically 5 mm behind the point of impact of the welding beam along the axis of welding but the actual field of view is only 3 mm behind said point of impact, the interpretation of the thermal image in terms of the maximum temperature may result in one unwarrantedly reducing the power of the beam, thereby leading to the delivery of welded plates in which the weld is of poor quality. If the actual field of view of the camera is shifted transversely to the welding axis compared to the supposed field of view, the thermal profile will be shifted and will be interpreted as a shifting of the welding axis with respect to the plane of the joint of the plates being welded. Corrective shifting of said welding axis will not restore the welding axis as supposed but rather will shift the actual welding axis to a laterally incorrect locus even though the thermal profiles signaled by the camera will now be interpreted as perfectly centered.
Thus, it is important to have full and accurate control of the position of the field of view of the camera with respect to the point of impact of the welding beam. This can be achieved by achieving accurately reproducible regulation of the position of the thermographic IR camera relative to the welding head; i.e. such that said relationship is preserved with each regulative action.
One may consider regulating both the position of the welding head and the position of the camera with respect to a reference point fixed in space, e.g. a point on the housing of the welding machine. In practice, such a reference point and frame of reference are indeed reliable in providing a means of regulating the relative position of the welding head and the camera; however, they cannot guarantee constancy of the exact position of the zone of the metal plates observed by the camera relative to the point of impact of the welding beam on said plates, because of variabilities due to tolerances in the dimensions of various components which are components of the camera and of the welding head, and because of possible time wise variations in the optical paths of these apparatuses.